1. Field of the Invention
The invention relates in general to an apparatus for converting a digital signal to a corresponding analog signal and a method thereof, and more particularly, to the apparatus set in the liquid crystal display for executing gamma correction and the method thereof.
2. Description of the Related Art
Featuring the favorable advantages of thinness, lightness, and generating low radiation, liquid crystal displays (LCDs) have been widely used. The LCD panel includes a number of pixels and the light transmittance of each pixel is determined by the difference between the upper plate voltage and the lower plate voltage.
FIG. 1 shows the gamma relation between the gamma voltage and the luminance of the pixel. The X-axis represents the lower plate voltage and the Y-axis represents the light transmittance of the pixel. When the magnitude of the upper plate voltage is fixed, the difference between the upper plate voltage and lower plate voltage is determined by the magnitude of the lower plate voltage. The corresponding relation between the lower plate voltage and the light transmittance of the pixel is nonlinear, as shown by the gamma curve in FIG. 1, wherein the difference between the upper and lower plate voltage is called the gamma voltage. The gamma curve shown in FIG. 1 includes a positive polarity gamma curve 102 and a negative polarity gamma curve 104. Each light transmittance of the pixel corresponds to a positive polarity gamma voltage signal and a negative polarity gamma voltage signal. The luminance of the pixel can be determined by the light transmittance of the pixel. Therefore, the luminance of the pixel can be controlled by controlling the magnitude of the gamma voltage according to the gamma curve.
FIG. 2 shows the block diagram of a nonlinear digital-to-analog converter (DAC) 202. The driving circuit of the liquid crystal display includes a nonlinear digital-to-analog converter 202 for converting the digital pixel signal (DATA) to the corresponding analog gamma voltage signal (OUT). Since the relation between the luminance of the pixel and the gamma voltage is not linear, the corresponding relation between the digital pixel signal (DATA) and the luminance of the pixel then is approximated as linear by executing the gamma correction according to the gamma curve. This process is called gamma correction.
FIG. 3 shows the ideal relation between each digital pixel signal and the corresponding light transmittance of the pixel. Each digital pixel signal is an eight-bit binary signal. Thus, there are 256 digital pixel signals for representing 256 gray level luminance of the pixel respectively. Through executing gamma correction using the nonlinear digital-to-analog converter 202, the relation between each digital pixel signal and the corresponding light transmittance of the pixel can be linear, as shown by the solid line in FIG. 3.
FIGS. 4A˜4C show the method of performing the gamma correction. In FIG. 4A, the points A, B, C, D, and E chosen from the positive polarity gamma curve 402 and the points A′, B′, C′, D′, and E′ chosen from the negative polarity gamma curve 404 are specific reference points. According to the gamma curve shown in FIG. 4, each reference point corresponds to a reference gamma voltage signal (V0˜V9). Each positive polarity gamma voltage signal (V0, V1, V2, V3, and V4) and the corresponding negative polarity gamma voltage signal (V9, V8, V7, V6, and V5) correspond to the same reference digital pixel signal (0, 63, 127, 191, and 255) respectively, as shown in FIG. 4A. Since the relation between each gamma voltage signal (V0˜V4 and V9˜V5) and the light transmittance of the pixel (T0˜T4) is the nonlinear gamma curve shown in FIG. 4B, the relation between each reference digital pixel signal (0, 63, 127, 191, and 255) and the corresponding reference gamma voltage signal (V0˜V4 and V9˜V5) cannot be linear but is like the gamma curve shown in FIG. 4A instead. In this manner, the relation between each digital pixel signal (0, 63, 127, 191, and 255) and the corresponding light transmittance of the pixel (T0, T1, T2, T3, and T4) respectively can be linear, as shown in FIG. 4C. When performing the gamma correction, the nonlinear DAC converts each digital pixel signal to the corresponding gamma voltage signal according to the relation between the reference digital pixel signal and the corresponding reference gamma voltage signal.
The conventional nonlinear digital-to-analog converter 202 includes two strings of resistors. Each resistor string includes 255 resistors. Furthermore, each resistor string includes five input nodes (V0˜V4, V5˜V9) for receiving the reference gamma voltage signals and 256 output nodes for outputting the gray level voltage signals. When the gamma correction is executed, the gamma output voltage signal corresponding to the digital pixel signal can be determined according to the gray level voltage signals.
FIGS. 5A˜5C show three different gamma curves related to the colors red, blue, and green. The color liquid crystal display includes three different kinds of pixels for displaying red, blue, and green respectively, and the three gamma curves are marked “R”, “G”, and “B” for each respective color, as shown in FIG. 5A. Each color corresponds to the specific gamma curve. According to the “R”, “G”, and “B” gamma curves, the gamma voltages corresponding to the maximum luminance of the pixels are VRM, VBM, and VGM for red, blue, and green respectively. The magnitude of VBM is smaller than that of VGM, and VGM is smaller than VRM, i.e. VBM<VGM<VRM. When executing the conventional gamma correction, the maximum magnitude of the gamma voltage signal outputted from the conventional nonlinear digital-to-analog converter is set to be VBM and all other gamma voltage signals corresponding to the digital pixel signals are determined according to the magnitude Of VBM. Therefore, the relation between each digital pixel signal and the corresponding gamma voltage signal is fixed regardless of the displaying color of the pixel corresponding to the digital pixel signal.
For example, after setting the maximum gamma voltage signal to be VBM, the relation between each of the digital pixel signals 0, 63, 127, 191, and 255 and the corresponding gamma voltage signal V0, V1, V2, V3, and V4 respectively can be determined so as to maintain the linear relation between each digital pixel signal and the corresponding light transmittance of the pixel, as shown in FIGS. 4A˜4C and FIG. 5A. However, the gamma curves for green, red, and blue are all different. If the relation between each of the digital pixel signals 0, 63, 127, 191, and 255 and the corresponding gamma voltage signal V0, V1, V2, V3, and V4 respectively is determined according to one of the three gamma curves, and for the digital pixel signals for displaying the other two colors, the relation between each digital pixel signal and the corresponding light transmittance of the pixel cannot be linear, as shown in FIGS. 5B and 5C. Also, if the displaying color of the pixel is red or green, the luminance of the pixel cannot be at its maximum luminance since the maximum magnitude of the gamma voltage signal is set to be VBM, and VBM<VGM<VRM. Therefore, the displaying performance of the display panel is degraded.